Electric power leveling technologies draw attention from the standpoint of reducing carbon dioxide emissions, installation costs, and electric rates. The electric power leveling techniques based on a power storage device cause the power storage device to be charged during a low electric-rate time band such as midnight hours. During peak electric energy consumption hours in daylight time, electric energy is discharged by the power storage device instead of being supplied from a utility power supply. As a result, a peak value of supplied electric energy from the utility power source (amount of electric energy consumption) is lowered.
The use of electric energy charged during low electric-rate time bands in midnight is expected to lower electric rate. A lowered peak value of the electric energy consumption leads to a reduction in the operation opportunity of a heat power plant, and is expected to reduce carbon dioxide emissions.
If a plurality of power consuming loads (such as home electronics, and personal computers) are connected to a single power storage device to level power, the power storage device needs to be a high-capacity one. Construction work is to be performed in order to enable each power consuming load to be connected to a high-capacity power storage device. Costs for the construction work become high.
Electric power leveling systems including a plurality of power storage devices have been studied (as described in Japanese Laid-Open Patent Publication No. 2001-258176, Japanese Patent No. 3730614, and Japanese Laid-Open Patent Publication No. 2007-336796). A plurality of power consuming loads are divided into several groups, and a power storage device is arranged on a per group basis. Charge and discharge operations of each power storage device are controlled and power of the entire system is leveled with a plurality of small-capacity power storage devices. A small-capacity power storage device is easy to add and remove. Such an electric power leveling system provides excellent scalability in response to an increase or a decrease in the power consuming load.
The peak value of the electric energy supplied from the utility power source is lowered as much as possible in the power leveling process based on the plurality of power storage devices. Ideally, the power leveling process based on the plurality of power storage devices provides the same advantages as those provided when the power leveling process is performed with all the power consuming devices connected to a single high-capacity power storage device.
If the power storage devices are individually controlled, it is likely that the timing of the minimization of the remaining electric energy (stored electric energy) is greatly different from device to device.
FIG. 1 illustrates a transition of the stored electric energy of each power storage device with time when the power storage devices are power-level controlled on an individual basis. Transitions of the stored electric energy of the four power storage devices A-D with time are illustrated in FIG. 1. If the control process is performed on an individual basis, the timing of the minimization of the stored electric energy is greatly different from device to device.
If the timing of minimum stored electric energy is greatly different from device to device, the sum of the stored electric energy of all the power storage devices at each time fails to reach a sufficiently small minimum value.
FIG. 2 illustrates a transition of the sum of stored electric energy of all the power storage devices with time when the power storage devices are power-level controlled on an individual basis. The transition of the sum of stored electric energy of the four power storage devices A-D of FIG. 1 with time is illustrated in FIG. 2. A minimum value is reached at about 18:00 o'clock. The closer to zero the minimum value of the stored electric energy, the better the minimum value. The minimum value illustrated in FIG. 2 is far from zero. This suggests that the power storage devices are not sufficiently used in comparison with the power leveling process with the single high-capacity power storage device used.
The ideal control described above is difficult to perform even if the power storage devices are individually controlled in an optimum fashion.